Method for regulating a compressed air supply system of a motor vehicle

ABSTRACT

A method for regulating a compressed air supply system of a motor vehicle, the system having a driving engine for driving the motor vehicle, which can be switched via a switchable hydrodynamic coupling to a driven connection with an air compressor, so that the air compressor feeds a compressed air system via an air delivery side.

The invention relates to a method for regulating a compressed air supply system of a motor vehicle, the system comprising a driving engine, which is the driving engine of the motor vehicle, that is, which serves for locomotion of the vehicle, as well as an air compressor, which feeds a compressed air system, and a hydrodynamic coupling, which is connected in the driven connection between the driving engine and the air compressor. The hydrodynamic coupling can be filled and emptied in order to switch the air compressor on and off, depending on the pressure state in the compressed air system. The air compressor is designed, in particular, as a reciprocating piston air compressor.

Compressed air supply systems, as they relate to the invention, have the advantage that, by means of the intervening hydrodynamic coupling, on the one hand, an energetically favorable switching off of the air compressor is possible when a feeding of the vehicle compressed air system is not necessary in the system on account of an adequate pressure level, this being accomplished by “simply” emptying the hydrodynamic coupling. On the other hand, by means of the intermediate connection of the hydrodynamic coupling between the driving engine and the air compressor, a vibration damping is effectively achieved and it is reliably prevented that, when a reciprocating piston air compressor is used, a negative torque, that is, a torque produced by the compressor, which can arise in the region of the upper dead point of the reciprocating piston air compressor, is transmitted back from the reciprocating piston air compressor to the driving engine or to a gearbox connected to it.

In such a compressed air supply system, there exist a plurality of states with different boundary conditions, in which the different components of the compressed air supply system must be suitable for working together in such a way that an operation that is energetically favorable and protective to components is ensured. Mentioned as different boundary conditions, for example, are the driving of the vehicle versus the standing still of the vehicle, the road profile that the vehicle is in the process of traveling, such as climbing or descending a hill, as well as different pressure states in the compressed air supply system of the vehicle—for example, above a maximum allowable pressure, below the minimum allowable pressure, and below a so-called triggering pressure, at which the spring-loaded actuator in the vehicle braking system is triggered and below which the vehicle must not drive. This trigger pressure ensues through the design of the brakes as “fail-safe”; that is, in the event of a failure of the compressed air system, pressure is applied to the brake shoes, for example, by means of springs, so that braking results. At and above a certain air pressure, the brake shoes can then be actively triggered, so that, below this pressure, the vehicle cannot drive off.

Reference is made to the following documents in regard to the prior art:

-   -   GB 775,525 A     -   U.S. Pat. No. 6,036,449 A     -   U.S. Pat. No. 4,459,085 A

The invention is based on the problem of presenting a method for regulating such a compressed air supply system, which ensures an especially efficient and protective operation of all components of the compressed air supply system and which contributes to an energetically favorable driving mode of the vehicle.

The inventive problem is solved by a method that has the features according to claim 1. The subclaims describe advantageous and appropriate further developments of the inventive method.

According to the simplest embodiment of the inventive method, the pressure in the compressed air system, which, when the pressure drops below a certain minimum pressure, for example, due to a corresponding high consumption of various consuming units connected to the compressed air system or due to leaking, is to be filled with compressed air, is registered and compared with a predetermined minimum value. When the registered pressure lies below the predetermined minimum value and the air compressor, by means of which compressed air is to be fed into the compressed air system, is not in operation, then the hydrodynamic coupling is filled with working medium so that rotary power is transmitted from the driving engine via the hydrodynamic coupling to the air compressor. Filling of the hydrodynamic coupling is understood to mean a filling of the working chamber of the hydrodynamic coupling, which, as is known, is formed by the pump impeller and by the turbine wheel, the working medium in the working chamber transmitting a torque from the pump impeller to the turbine wheel.

At the moment when the hydrodynamic coupling is filled with working medium, the air compressor begins its compression activity, the states in the air compressor during this starting process being strongly dependent on the counterpressure against which the air compressor operates.

The inventive method leads to the fact that the air compressor is constantly driven against a small counterpressure, which is especially protective for the air compressor and excludes a “forcing” of the driving engine, that is, a reduction in the rpm of the driving engine. To this end, in accordance with the invention, when the air compressor is started, the air delivery side of the air compressor is first connected, depending on a given criterion, with the surroundings, in which, as is known, a comparatively low ambient pressure prevails, or is connected with a low-pressure system. Low-pressure system is understood here to mean a pressure system in which a lower pressure prevails than in the compressed air system that is to be fed. For example, such a low-pressure system may have a maximum pressure of 2 bars. Obviously, it is also possible, when several low-pressure systems—for example, various pressure tanks—are available, to select during each starting operation a suitable low-pressure system with which the air delivery side of the air compressor is connected.

Only when the air compressor has reached a certain rpm is its air delivery side connected with the compressed air system that is to be fed by it. It can be determined directly or indirectly when a suitable rpm has been reached. According to one embodiment of the invention, the rpm of the air compressor or of a component part in a driven connection with it is registered and compared with a given set value. Once the set value has been reached, the connection of the air delivery side with the surroundings or with the low-pressure system, respectively, is switched over to a connection with the compressed air system to be fed.

According to a second embodiment, the rpm of the air compressor is taken into consideration indirectly for switching over to the compressed air system in that, between the beginning of the filling operation of the hydrodynamic coupling and the switching over of the air delivery side to the compressed air system, a certain predetermined interval of time elapses.

According to another embodiment of the invention, it is decided, depending on the registered pressure in the compressed air system, whether the air compressor is to begin immediately starting to supply the compressed air system, namely, when a correspondingly low pressure prevails in the compressed air system to be supplied—for example, when a pressure of less than 2 bars prevails—or whether the air delivery side is first connected with the surroundings or with a low-pressure system, so that the air compressor feeds air into the surroundings or into the low-pressure system, respectively, which is separated from the compressed air system.

The pressure value that is used for the decision on a direct feeding of air by means of the air compressor into the compressed air system is designated here as the maximum counterpressure, because a start-up of the air compressor against a pressure above this maximum counterpressure would create a load on the air compressor or on the components that drive the air compressor—for example, the driving engine—and this is to be avoided.

The air delivery side of the air compressor can be connected with the surroundings or with a low-pressure system, respectively, or with the compressed air system that is to be fed by means of the corresponding setting of a relay valve, particularly through application of a control pressure on a 3/2-directional control valve, advantageously through application of an air pressure that is delivered out of the compressed air system.

It is also possible to introduce a relay valve into the connection conducting working medium between a working medium source and the hydrodynamic coupling, by means of which working medium is carried out of the working medium source into the working chamber of the hydrodynamic coupling, this relay valve being designed, in particular, as a 2/2-directional control valve and effecting an opening and closing of the connection that conveys working medium in order to control the filling of the hydrodynamic coupling. When the hydrodynamic coupling is to be filled, the valve switches correspondingly to its opened position, whereas it switches to its closed position when the hydrodynamic coupling is to be emptied.

It is especially advantageous to introduce, into the above-mentioned connection conducting working medium, a 2/3-directional control valve, which, in addition to the opened state and the shut-off state, has a third switched state. This third switched state is a state in which the cross section of the connection conducting working medium to the hydrodynamic coupling is decreased, so that a throttling of the flow of working medium into the hydrodynamic coupling ensues. This state is therefore also referred to as the throttled position of the valve and is set precisely in the case when, although air pressure is to be fed into the compressed air system by means of the air compressor, the air compressor operates at an overspeed, that is, at an rpm that is above an allowable rpm. In order to decide whether the 2/3-directional control valve is to be switched to the throttling state, the rpm of the air compressor is registered, either directly or else via the rpm of a component part in driven connection with the air compressor.

The 2/3 directional control valve is switched to its opened (fully opened) state when the registered pressure in the compressed air system lies below the given minimum value, that is, when air is to be fed into the compressed air system by means of the air compressor and the air compressor operates at an allowable rpm, that is, not an overspeed. The 2/3-directional control valve is switched to its closed position when the registered pressure in the compressed air system lies above the given maximum value, that is, when air is no longer to be fed into the compressed air system by means of the air compressor.

When the introducing of working medium into the hydrodynamic coupling is interrupted, because the registered pressure in the compressed air system lies above the pre-given maximum value, the air delivery side of the air compressor is advantageously connected at the same time with the surroundings or with a/the low-pressure system. In order to prevent an overspeed of the air compressor, which could arise through the abrupt drop-off in the counterpressure during a switching over of this air-conducting connection, the switching over of this connection occurs advantageously with a time delay after the shut-off of the feeding of working medium to the hydrodynamic coupling, that is, after the beginning of emptying the hydrodynamic coupling.

According to an advantageous configuration of the inventive method, this method accomplishes a safety function, by means of which it is recognized that when the vehicle is moved, although the pressure in the compressed air system has not reached a pre-given second minimum value, which lies below the first pre-given minimum value and represents the above-described trigger pressure of the spring-loaded actuator, even though the air compressor is in operation. This registered state means that the rpm of the air compressor is not adequate to maintain a sufficient pressure in the compressed air system. In accordance therewith, the rpm of the driving engine is increased in response, so that the rpm of the air compressor increases.

In order to exert an additional braking effect on the driving engine of the vehicle when the vehicle is in engine overrun, which, as is known, is set when, during descending a hill, rotary power is transmitted from the wheels of the vehicle onto the engine driven shaft—in the case of an internal combustion engine, onto the crankshaft—the hydrodynamic coupling is advantageously constantly filled during engine overrun of the vehicle, so that the air compressor is driven by the driving engine. When, in this way, the registered pressure in the compressed air system rises above the pre-given maximum value at the same time, the compressed air from the compressed air system is vented—for example, through a safety valve—so that the pressure in the compressed air system drops once again or at least is limited to a maximum value. Due to the fact that driving power is transmitted from the driving engine to the air compressor, which operates against the high counterpressure of the compressed air system, it is possible, in addition, to bring about a braking effect on the engine, which leads to a relief of the parking or of the friction brakes of the vehicle.

In order to enhance this additional braking effect on the driving engine of the vehicle when the vehicle is in engine overrun, a valve or a throttle can be introduced into an air line that is connected to the air delivery side of the air compressor, this valve or throttle interrupting or throttling the air flow on the air delivery side of the air compressor. Both lead to an increase in the counterpressure against which the air compressor operates and thus to an increased power uptake of the air compressor. In particular, the above-described relay valve, which is used for switching over the air delivery side of the air compressor to the surroundings or to a low-pressure system, respectively, or to a compressed air system that is to be fed, can have a further switched step, in which it throttles or blocks the flow in the line on the air delivery side.

According to another advantageous embodiment of the invention, the topography of the road traveled by or to be traveled by the motor vehicle enters into the inventive method. In the case of known topography, which is registered, in particular, by use of a so-called vehicle navigation system, the pre-given minimum value, above which the air compressor is to be started, varies or else, depending on this topography, a third minimum value is given in advance, which is smaller than the first minimum value and, in particular, lies between the first and the second minimum values. This third minimum value represents the allowable minimum value of the registered pressure in the compressed air system above which the air compressor is not yet started when the motor vehicle climbs a hill and, on account of the known topography, it is ascertained that no great braking power is to be anticipated in the course of the next few kilometers, for example, on account of a steep downward slope. When, for example, the motor vehicle travels up a hill and, prior to reaching the crest of the hill, the first minimum value is not attained, but this is not so for the third minimum value, and it is also known that, after the crest is reached, no steep downward slope requiring a high braking power follows, then the regulation method allows a drop-off of the pressure in the compressed air system down to the pre-given third minimum value. This has the advantage that the vehicle engine, which has to furnish a high power output on account of the requisite conveyance of the vehicle up the hill, is not loaded additionally by the driving of the compressor.

In particular, in addition to the road-specific data registered by means of the navigation system, stored travel-specific data from earlier travels of the vehicle on this road or in similar situations can enter into the known topology and be employed for setting the pre-given third minimum value.

The invention will be described in greater detail below on the basis of embodiment examples and the associated figures.

Shown are:

FIG. 1 a state that is regulated by the inventive method during start-up of the driving engine when the compressed air system is largely empty;

FIG. 2 a state that is regulated by the inventive method after the start-up of the engine when the pressure in the compressed air system lies in a desired pressure range;

FIG. 3 an embodiment of a compressed air supply system that is regulated by a method according to the invention, the air compressor being protected against an overspeed;

FIG. 4 an embodiment of a compressed air supply system with constantly regulating 3/2-directional control valve.

Seen in FIG. 1 is the driving engine 1, which is in driven connection with the drive shaft 2.1 of a hydrodynamic coupling 2 via a gearbox 4. The hydrodynamic coupling 2 is in driven connection through its output side 2.2 with an air compressor 3, which feeds a compressed air system 7 via an air delivery side 3.1. The compressed air system 7, for example, can be a compressed air tank, from which compressed air can be withdrawn via one or more corresponding outputs in order to supply consuming units.

The hydrodynamic coupling 2 has a connection for introducing working medium 5 from a working medium reservoir (not depicted) as well as a connection 6 that carries off working medium and by means of which working medium can be transported out of the hydrodynamic coupling 2. Coming into consideration as working medium of the hydrodynamic coupling 2, for example, is oil or water or a mixture containing one or both of these substances.

In the air-conducting line between the air compressor 3 and the compressed air system 7 is a 3/2-directional control valve (relay valve 8), which, in the position shown in FIG. 1, creates an air-conducting connection between the air delivery side 3.1 of the air compressor 3 and the compressed air system 7, so that, during driving of the air compressor 3, compressed air is fed into the compressed air system 7. In this state, the 3/2-directional control valve is free of applied pressure due to a control pressure, which, as is known, can be applied to one side of such a relay valve, also referred to as a pilot valve, and operates against a compression spring on the other side of the valve.

This state of the 3/2-directional control valve 8, shown in FIG. 1, is then set when the pressure in the compressed air system 7 lies below the pre-given minimum value, that is, when compressed air is to be fed into the compressed air system 7 by means of the air compressor 3 in order to raise the pressure in the compressed air system 7.

Placed in the connection 5 for conducing working medium, through which the working chamber of the hydrodynamic coupling 2 can be filled with working medium, is a 2/2-directional control valve (relay valve 9), on one side of which may also be applied control pressure, which is opposed to the pressure on a compression spring disposed on the opposite-lying side. Accordingly, the 2/2-directional control valve may be switched between a (fully) opened state, and a shut-off state depending on the magnitude of the control pressure.

Shown in FIG. 1 is the state in which, for example, the driving engine 1 has been started and the air tank is empty or the compressed air system has a pressure that is too small or has the ambient pressure, that is, the registered pressure in the compressed air system lies below the pre-given first minimum value. Provided that the pressure in the compressed air system 7 lies below the described second minimum value as well, that is, the so-called trigger pressure, it is necessary, first of all, to use the air compressor 3 to pump so much compressed air into the air system 7 that the pressure of the compressed air system 7 exceeds the trigger pressure, that is, the pre-given second minimum value, before the vehicle may drive off. To this end, if need be, the engine rpm of the driving engine is increased, for example, to 1200 rotations per minute or more in order to attain a corresponding rpm of the air compressor 3 and a corresponding conducting capacity of the air compressor 3.

Due to the fact that the air tank is empty or the pressure in the compressed air system 7 is markedly low, the air compressor 3 can be driven directly against the pressure in the compressed air system 7; that is, the relay valve 8 is switched directly to the connected state or, simultaneously with the switching of the relay valve 9, to the connected state.

When the driving engine is started and the registered pressure value in the compressed air system 7 lies above the pre-given minimum value or when, after the start-up of the air compressor 3, the pressure in the compressed air system 7 exceeds the pre-given maximum value, the valve 8 is switched to its shut-off state by applying a control pressure on the relay valve 8, which opposes the pressure of the compression spring on the opposite-lying side (from top to bottom in the figures) and which switches the relay valve 8 to the state in which the feeding connection to the compressed air system 7 is closed and the air delivery side of the air compressor 3 is connected with the surroundings.

At the same time, a control pressure is applied to the relay valve 9 and switches it to its shut-off state, so that no working medium is carried any longer to the hydrodynamic coupling 2. The working medium present in the hydrodynamic coupling 2 is carried off via the line 6 so that the hydrodynamic coupling 2 is emptied.

In order to prevent the air compressor 3 from reaching its overspeed range, because the counterpressure is dropped by switching over the valve 8 and substantial torque is still transmitted at the same time from the driving engine 1 to the air compressor 3 through a correspondingly high filling level of the hydrodynamic coupling 2, the switching of the valve 8 to its bypass position (FIG. 2) can occur with a time delay after the switching of the valve 9 to its shut-off position. A danger of increase in rpm of the air compressor 3 exists particularly when the oil volume between the relay valve 9 and the hydrodynamic coupling 2 is large, so that the air compressor 3 continues to run for a correspondingly long time after the switching of the valve 9 to the shut-off position. The shorter the “pumping interval,” that is, the more often the air compressor 3 is switched on and off in a pre-given time period, the stronger the effect of losses due to switching off or the longer the subsequent running times. By means of a suitable switching sequence, these effects can be compensated for or at least brought under control.

In FIG. 3, the relay valve 9 takes the form of a 2/3-directional control valve. This 2/3-directional control is switched to the throttled position, which is additionally provided for relative to the embodiment shown in FIGS. 1 and 2, when, although an air compressor operation is desired because the pressure level in the compressed air system 7 lies below the minimum allowable value or has not yet reached the maximum allowable value, the air compressor 3 operates at too high an rpm. In this case, the inflow of working medium into the hydrodynamic coupling 2 is throttled, so that the rpm of the turbine wheel or of the output side 2.2 of the hydrodynamic coupling 2 and thus also the rpm of the air compressor 3 is reduced. The hydrodynamic coupling is brought here into a state with a partial filling, by means of which the slip created in the coupling 2 between the pump impeller and the turbine wheel is increased, so that, at constant driving rpm, the driven rpm is lowered.

In FIG. 4, the relay valve 9 takes the form of a constantly regulating 3/2-directional control valve. By means of this constantly regulating valve, the inflow of working medium into the hydrodynamic coupling 2 is constantly adjusted by diverting or conducting off a desired quantity of the medium flowing into the relay valve 9 by means of the relay valve 9 and conducting only the “remainder” of the working medium to the hydrodynamic coupling 2. Obviously, it is possible to set the proportion carried away to zero so that the entire quantity of working medium fed to the relay valve 9 is also conducted to the hydrodynamic coupling 2. It is also equally possible to carry off the entire quantity of working medium fed to the relay valve 9 so that no working medium is conducted to the hydrodynamic coupling 2.

The relay valve 9 can have two switched positions for three connections (that is, it can be designed as a 3/2-directional control valve); in a first switched position, the flow of working medium to the hydrodynamic coupling 2 is interrupted and, in the second switched position, the quantity of flow of working medium flowing into the hydrodynamic coupling 2, as depicted, is regulated. Obviously, it is also possible to provide a constantly regulating valve, which is not designed as a relay valve, but rather also brings about the complete interruption of the flow of working medium into the hydrodynamic coupling 2 by having the working medium flowing into the valve 9 completely bypass the hydrodynamic coupling 2 without it being switched to a second position for this purpose.

Some boundary conditions that can be used in the inventive method as limiting values for the regulated variables will be mentioned below by way of example:

-   -   Switch-off pressure: p_(tank, max)=12.5±0.3 bars (pre-given         maximum value in the compressed air system)     -   Switching range: Δp=1.7±0.3 bars     -   Switch-on pressure: p_(tank, on)=10.8±0.6 bars (pre-given         minimum pressure value in the compressed air system)

Motor vehicle braking system 10 bars Additional consuming circuit 10 bars (trailer 8.5 bars) Legally required filling period max. 12 min

-   -   Spring-loaded actuator triggering pressure p_(L)=5.5±0.3 bars         (pre-given second minimum pressure value; above this tank         pressure, the vehicle can drive off) 

1.-13. (canceled)
 14. A method for regulating a compressed air supply system of a motor vehicle, comprising: registering a pressure in the compressed air supply system; comparing the pressure with a selected first minimum value; filling a hydrodynamic coupling with a working medium when the pressure lies below the selected first minimum value and starting an air compressor; connecting an air delivery side of the air compressor with the surroundings or with a low-pressure system for a selected period of time, depending on a revolutions per minute of the air compressor or of the hydrodynamic coupling and/or depending on the pressure; and connecting the air delivery side of the air compressor with the compressed air supply system after conclusion of the selected period of time, when a selected revolutions per minute is reached and/or when the pressure is below a selected pressure value of a maximum counterpressure that is smaller than a selected first minimum pressure.
 15. The method according to claim 14, further comprising: comparing the pressure with the selected pressure value during start-up of the air compressor; and connecting the air delivery side of the air compressor to the compressed air system when the pressure is equal to or smaller than the maximum counterpressure so that a feeding into the compressed air system ensues.
 16. The method according to claim 14, further comprising: comparing the pressure with the selected pressure value during start-up of the air compressor; and connecting the air delivery side of the air compressor with the surroundings or with a low-pressure system when the pressure is greater than the maximum counterpressure.
 17. The method according to claim 14, further comprising: disposing a relay valve in an air-conducting line between the air compressor and the compressed air system; and switching the relay valve so that the air compressor feeds compressed air into the compressed air system.
 18. The method according claim 14, further comprising connecting a relay valve to the hydrodynamic coupling so that a working medium can be introduced into a working chamber of the hydrodynamic coupling, the relay valve opening and closing a connection so that filling of the hydrodynamic coupling can be controlled.
 19. The method according to claim 18, further comprising emptying the hydrodynamic coupling so that the air compressor no longer feeds air into the compressed air system when the pressure lies above the selected maximum value.
 20. The method according to claim 19, further comprising: introducing a 2/3-directional control valve into the connection; switching the 2/3-directional control valve to a first state in which the connection is opened when the pressure in the compressed air system lies below the selected first minimum value and the air compressor is operating at a revolutions per minute below or equal to a maximum allowable speed; switching the 2/3-directional control valve to a second state in which the 2/3-directional control valve is shut-off when the pressure lies above the selected maximum value; and switching the 2/3-directional control valve to a third state in which the 2/3-directional control valve is in a throttling position when the pressure lies below the selected maximum value and the air compressor operates at a revolutions per minute above a maximum allowable revolutions per minutes.
 21. The method according to claim 20, further comprising interrupting the connection when the pressure lies above a selected maximum pressure.
 22. The method according to claim 21, wherein the interrupting step comprises switching a position of the relay valve.
 23. The method according to claim 21, wherein the interrupting step comprises a time delay after the beginning of emptying of the hydrodynamic coupling.
 24. The method according to claim 14, further comprising: registering a speed of the motor vehicle; and comparing the pressure with a selected second minimum value that lies below the selected first minimum value, the second minimum value representing a spring-loaded actuator triggering pressure of a braking system of the motor vehicle above which the motor vehicle must travel at a speed greater than zero, the pressure being below the selected second minimum value, wherein the revolutions per minute of a driving engine is increased.
 25. The method according to claim 24, further comprising: constantly filling the hydrodynamic coupling with the working medium when rotary power is transmitted from wheels of the motor vehicle to an engine driven shaft; and venting compressed air from the compressed air system when the pressure exceeds the selected maximum value in order to limit the maximum pressure.
 26. The method according to claim 25, further comprising increasing the pressure on the air delivery side to a selected value by a shut-off valve or a throttle.
 27. The method according to claim 14, further comprising: registering or storing in memory a topography of a road on which the motor vehicle travels; setting the selected minimum value or a selected third minimum value depending on the topography; and starting the air compressor by filling the hydrodynamic coupling when the selected minimum value is not reached and when, simultaneously, the selected third minimum value is not reached.
 28. The method according to claim 27, wherein the setting step comprises setting the selected minimum value or a selected third minimum value depending on road- and travel-specific data recorded and stored in memory during earlier travels of the motor vehicle. 